In a liquid crystal display (hereinafter “LCD”) panel, a backlight having multiple lamps such as cold cathode fluorescent lamps (hereinafter “CCFL”s) is used to provide illumination. Usually, these lamps are individually driven by power conversion stages including drivers and transformers. FIG. 7 shows a conventional backlight driving structure, where driver 1, driver 2, . . . , driver N are attached to a printed circuit board (hereinafter “PCB”) to drive lamps CCFL-1, CCFL-2, . . . , CCFL-N of the backlight, respectively, where N is an integer. For a large LCD panel, more lamps are needed in the backlight for providing sufficient illumination to the LCD panel. However, as the number of lamps is increased, the number of driving components of the backlight is increased accordingly, which adds up to a higher cost and a larger mechanical size. Furthermore, each of the power conversion stages operates at different frequencies. Such non-synchronous operation tends to result in a mutual interference, and more seriously, it may interfere the video signals of the LCD panel and result in ripple noises on the screen.
In order to reduce the cost of backlights, a balance circuit can be employed to allow a single driver to drive multiple lamps. FIG. 8 shows a conventional backlight driving structure using a balance circuit identified as “Cell.” In the backlight driving structure, each of driver 1, driver 2, . . . , driver N is used to drive a pair of lamps and a balance circuit Cell is adapted for balancing lamp currents of the lamps CCFL-1, CCFL-2, . . . , CCFL-2N-1, CCFL-2N. Different types of balance circuit (Cell) 901, 902 and 903 are shown in FIG. 9. Typically, a balance circuit includes capacitors, inductors, and/or transformers. All these capacitors, inductors and transformers are passive components. Because of intrinsic limitations of the passive components, the more the passive components are used, the larger the errors in the balance circuit are. Additionally, the passive components are unable to self-adjust their parameters, thus the properties of the lamps are sensitive to their surrounding environment. When drivers operate at different frequencies from a pre-designed frequency, operating parameters of the passive components need to be re-adjusted. The use of the passive components in the balance circuit thus may limit balancing effects of lamp currents in a backlight.
Alternatively, a current balance circuit using active components such as transistors, diodes and comparators is disclosed in U.S. Pat. No. 6,420,839 to Chiang et al., which is incorporated herein by reference in its entirety. As shown in FIG. 10, a current balance circuit 20 comprises a capacitor Cx seriesly connected to a slave lamp Lps, a first transistor Qp and a second transistor Qn with their collectors and emitters respectively coupled to the two ends of the capacitor Cx, a first diode Dp and a second diode Dn respectively coupled to the collector/emitter of the first transistor Qp and the second transistor Qn, and a comparator 22 having two inputs respectively connected to the sampling resistors Rm and Rs and one output connected to the bases of the first transistor Qp and the second transistor Qn. By using sampling resistors Rm and Rs, the current values Im and Is of the master lamp Lpm and the slave lamp Lps are converted into voltage values Vm and Vs, which are respectively fed to positive and negative inputs of the comparator 22. If Vm>Vs, i.e., the current Im passing through the master lamp Lpm is greater than the current Is passing through the slave lamp Lps, the comparator 22 outputs a high voltage (=Vref) and thereby drives the first transistor Qp and the second transistor Qn to discharge the capacitor Cx, so that the equivalent capacitive reactance of the capacitor Cx decreases, and thereby the current Is passing therethrough increases. If Vs>Vm, i.e., the current passing through the slave lamp Lps is greater than the current Im passing through the master lamp Lpm, the comparator 22 output a low voltage (=GND) and fails to drive the first transistor Qp and the second transistor Qn to discharge the capacitor Cx, so that the capacitive reactance of the capacitor Cx stays at the original value, the current Is passing therethrough decreases. The circuit balance circuit 20 is insensitive to the operating frequency and its surrounding environment. However, the transistors operate in its switching mode, thereby causing waveforms of the lamp currents nonsymmetrical. The nonsymmetrical current waveforms shorten the lifetime of the lamps. Additionally, two-bit outputs of a high and low voltage from the comparator result in inaccuracy in the lamp currents. Furthermore, the current balance circuit 20 as a long response time that may limit the performance of the backlight.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.